Field of the Invention
The present invention relates to a technique of properly reading a plurality of colorimetric patches formed on a colorimetric chart when image formation is adjusted in an image formation apparatus.
Description of the Related Art
An image formation system has been known in which a reading apparatus (output object reading apparatus) is connected to a latter stage of an image formation apparatus configured to form an image on a paper sheet such that the image on the paper sheet obtained as a result of the image formation is read by the reading apparatus. Similarly, an image formation apparatus has been known in which a reading unit (output object reading unit) is arranged on a downstream side of an image formation unit configured to form an image on a paper sheet such that the image on the paper sheet obtained as a result of the image formation is read by the reading unit.
The image formation apparatus such as a printer or a multi-functional peripheral is provided with an image adjustment mode. In this image adjustment mode, in order to enhance the quality of an output image, a colorimetric patch has been conventionally printed in color to provide a function of detecting this colorimetric patch by using an RGB color density sensor or the like to compare with original print data and, when a difference therebetween is observed, correcting print density and so on while forming an image.
Preferably, in order to decrease the number of paper sheets for the adjustment, the size of the colorimetric patch to be printed on the paper sheet during such an image adjustment mode is made as small as possible with respect to a paper sheet conveying direction such that a large number of the colorimetric patches can be formed on one paper sheet.
In recent years, in order to reduce an image adjustment period or the like for further enhancement of the image quality, an image reading apparatus including an image formation apparatus equipped with an inline image scanner has appeared. This image scanner can be used to correct a position shift between a front side and a rear side of a sheet, image density, a color tone, and the like in regard to the output image in real time.
Additionally, equipping an inline spectrophotometer for accurately measuring the color tone to correct an absolute color on print data can contribute to further reduction of the adjustment period. Because the image scanner can detect the patches on the entire surface of the paper sheet, by understanding a correlation between a true color tone acquired by the spectrophotometer and color data acquired by the image scanner to correct the image scanner, the patches can be actually formed on the entire surface of the paper sheet, whereby the paper sheet for the adjustment can be saved in turn. Alternatively, when the spectrophotometer alone is connected inline to the image formation apparatus such that the colorimetry is carried out on the patch on the paper sheet while being conveyed, effects of the period reduction and so on can be achieved as a consequence of automated adjustment.
In a case where the spectrophotometer is used, a configuration is employed in which reflected light of respective components of separated light is arithmetically calculated into L*a*b* color space data or XYZ color space data, whereby the color tone is deduced. The patch for which the colorimetry is to be carried out for a visible light source is irradiated by the spectrophotometer such that an optical spectrum of the reflected light is acquired. Additionally, in some cases, in order to exclude the influence of a fluorescent agent in the paper sheet for colorimetry and consequently obtain more accurate colorimetry, reflected light of an ultraviolet light source is acquired so as to be subjected to the arithmetic calculation.
Here, the size of a colorimetric patch P formed on a paper sheet P will be described with reference to FIG. 19. In a typical spectrophotometer, a lens unit configured to acquire the reflected light is approximately 4 mm, while a measurement diameter φ is also approximately 4 mm on the colorimetric patch.
Accordingly, a patch size PTsize_H of the colorimetric patch is required to have 4 mm+α per one colorimeter with respect to a paper sheet width direction (a direction perpendicularly intersecting the paper sheet conveying direction).
In addition, with respect to a paper sheet length direction (paper sheet conveying direction), the colorimetric patch is required to have a patch size PTsize_V=φ+mv+2 mg obtained by adding a movement distance my of the paper sheet P corresponding to a measurement period, the measurement diameter φ, and a margin mg.
The spectrophotometer is constituted by an exposure unit configured by an LED or the like, a diffraction grating configured to separate light, a CMOS sensor configured to detect the reflected light of the separated light, a signal output unit configured to output a detection result in accordance with a wavelength, and so on. The typical spectrophotometer uses the CMOS sensor or the like to collect the reflected light that has been separated in units of 10 nm in a range of 380 nm to 730 nm. Exposure by the LED or the like continues for a predetermined period during colorimetry. When the reflected light is not collected for the predetermined period, precise colorimetry cannot be achieved. In order to enhance the stability in repeated colorimetry, it is further preferable to take an integrated average of several times. The separated light reflection data obtained here is stored to a memory of the colorimeter and at the same time, subjected to arithmetic calculation processing into a colorimetric value in L*a*b* or the like. Results of these series of the processing are temporarily stored to a RAM and held until being transmitted to the reading apparatus.
Here, when a colorimetry period and the colorimetric patch on the moving paper sheet during colorimetry is considered in accordance with the typical spectrophotometer as a reference, the measurement diameter φ needs to be 4 mm, the colorimetry period needs to be 10 ms, and an analog amount of the reflected light needs to be equal to or larger than 1.3 ms when converted to a digital value. When a conveying speed of the paper sheet is assumed as 300 mm/s, a movement amount while the colorimetry is carried out one time is found out to be approximately 3.4 mm. Accordingly, the necessary patch size at least requires 7.4 mm, where 4 mm of the measurement diameter φ has been added. In other words, resolution for the colorimetry of the colorimetric patch is found out to be 7.4 mm in the paper sheet conveying direction.
Additionally, regarding the colorimetric patch described above, it is important to align a timing at the spectrophotometer between two adjacent colorimetric patches such that the colorimetry is carried out for each of the colorimetric patches without spanning across both of the colorimetric patches. As for techniques in this field, various types of relevant proposals are described in JP 2006-251652 A and JP 2000-39747 A.
(1) In JP 2006-251652 A, as illustrated in FIG. 20, a paper sheet conveying sensor 199 is controlled to detect a leading edge of a paper sheet while being conveyed, form a trigger signal with a predetermined timer, and carry out the colorimetry on each of colorimetric patches. In this case, due to deviations on an image formation side such as a blank space at the leading edge of the paper sheet (a distance from the leading edge of the paper sheet to the colorimetric patch) and printing magnification, a timing from the leading edge of the paper sheet to each of the colorimetric patches is to be slightly changed. Therefore, it is required to enlarge a patch size.
(2) JP 2000-39747 A employs a control approach in which a spectrophotometer judges a boundary of each of colorimetric patches by itself to start the colorimetry. In this case, a color difference (including differences in saturation and brightness) equal to or larger than a certain degree is required between respective patches. This accordingly causes a limitation on the arrangement of the colorimetric patches on a chart. In addition, an unavailable combination of the colorimetric patches has a possibility of occurring. Therefore, it is required to enlarge the colorimetric patch such that whether the boundary of each of colorimetric patches is certainly passed can be determined with ease.
(3) As illustrated in FIG. 21 and FIG. 22, another approach is also considered in which trigger patches are formed in advance so as to be synchronized with the colorimetric patches in the conveying direction on the paper sheet P such that a trigger sensor 190a detects the trigger patch and, in accordance with this detection of the trigger patch, a spectrophotometer 190b carries out the colorimetry on the colorimetric patch. In this case, the trigger patch serving as a reference point is prepared for each of the colorimetric patches and thus, it is made possible to precisely pinpoint the colorimetric patch while the colorimetry is carried out thereon almost without being affected by the blank space at the leading edge or printing magnification.
As described thus far, in the cases of (1) and (2), there has been a problem where the colorimetric patch is required to be enlarged. On the other hand, in the case of (3), the colorimetric patch is not required to be enlarged and it seems that the colorimetry can be precisely carried out on the colorimetric patch.
According to the verification by the inventors of the present application, however, it has been found that even the above-described approach (3) has problems caused during manufacturing, for example, a positional difference in the paper sheet conveying direction between the trigger sensor 190a and the spectrophotometer 190b (refer to FIG. 21) and a difference in detection directivity in the paper sheet conveying direction between the trigger sensor 190a and the spectrophotometer 190b (refer to FIG. 22).
For a remedy for this, a calibration is required in such a manner that, for example, a position where the trigger sensor is activated is measured with a scale while the trigger patch is moved in units of 0.1 mm and recorded for each of the sensors; thereafter, on the basis of the recorded positions, the positions of the spectrophotometer and the trigger sensor are aligned. Nevertheless, this requires a longer adjustment period and thus is not desirable.